Low-frequency amplifier



Sept. 19, 1961 H DE QUANT LOW-FREQUENCY AMPLIFIER Filed April 22, 1958 FIG.1

INVENTOR HANS DE QJANT United This invention relates to low-frequency amplifiers for speech or music oscillations comprising a device for volume control and a device for contrast variation control.

As is wellknown, radio-transmissions, gramophone records and the like show a reduced contrast with respect to the original reproduction, in order to enable the transmission path and the sound carrier to be used to better advantage.

In order to regain a musical performance as faithful as possible to the original, the low-frequency amplifier in the reproducing equipment needs a device for contrast increase (contrast expansion). To permit still a certain matching to the listeners personal wishes or to external conditions, it is preferable for the device for contrast increase to be made adjustable.

As is well'known, more particularly a wireless receiver is switched on not only for listening to a music piece with full attention. In many cases wireless and gramophone transmissions are also used as background noise, the listener then usually being preoccupied with other things. For this kind of transmissions. a low sound intensity is desirable. It is also possible that an interference, for example resulting from a telephone call, occurs during a music transmission of high quality of reproduction which is listened to with full attention and that it is desired temporarily to decrease the sound intensity of a reproduction to a low value. However, the receiver must then not be switched 0d" completely.

Experiments having resulted in the invention revealed that it is preferable with low sound intensity of reproduction to decrease also the contrast. In fact, the low sound intensity of reproduction is usually chosen such that loud portions of a strong original signal of a music piece or the like are but slightly predominant to the ambient. noise. if, in this case, the soft portions are considerably softer in accordance with the full-contrast or the reduced contrast as occurs with radio and gramophone record transmission, these soft portions are usually lost in the ambient noise, so that in practice the loudest portions only are still slightly audible, whilst the original soft portions are greatly disturbed.

These disadvantages are. avoided in a low-frequency amplifier for speech or music oscillations, comprising a device for volume control and a device for contrast variation control if, according to the invention, the two devices are coupled together and preferably so that a contrast decrease occurs upon adjustment to low sound intensity of reproduction and if desired a contrast increase occurs upon adjustment to high sound intensity of reproduction.

For the sake of clarity, it is. to. be noted that, on the one hand, the original signal transmitted has variations in amplitude which are determined by variations in the sound intensity of the transmission, these variations between the largest and smallest signals representing the extent of contrast, briefly the contrast, of the signal transmitted. On the other hand, the mean sound intensity of reproduction may be adjusted in the amplifier by control devices, which may also be regarded as an amplification control of the output signal with respect to the input signal. The amplification control brings about variation in the magnitude to be transmitted in the widest. sense and also comprises an. adjustment whereby theoutput than the input signal. In. connection with intensity and contrast as signal is weaker the coupled control of sound t es atom indicated by the invention, it is possible for characterizing the adjustment of amplification to use a mean value for the in ut signal or for the sound intensity of reproduction, which value in itself may be chosen arbitrarily.

It is also known in a circuit arrangement for contrast variation control to bring about a variation in amplification at the same time as the variation in characteristic, so that the maximum sound intensity of reproduction is maintained constant. Since the contrast or the extent of contrast is given by the ratio of maximum signal or maximum sound intensity of reproduction to minimum signal or minimum sound intensity of reproduction and at the receiving end the minimum sound intensity must havev a fixed value sufiiciently above the ambient noise or the like, variation in the extent of contrast does not occur in this known circuit arrangement. However, in an arrange ment according to the invention, it may be achieved that the sound intensity of reproduction associated with the smallest useful signal has an approximately constant value which sufficiently contrasts with the ambient noise. The soft portions of the reproduction then remain quite audible irrespective of the adjusted mean or maximum sound in tensity amplification) Upon adjustment to high sound intensity of reproduction (amplification), the normal reproduction or a reproduction with improved contrast is obtained with an am piifier according to the invention, whereas in the transmission adjusted to low soundintensity of reproduction (low amplification) the loud and soft portions of the original signal remain audible at least with approximately equal strength, it then being assumed that the signal strength is dependent only upon the sound intensity registered and transmitted, for example fluctuations of the high-frequency signal occurring in radio transmission may r be compensated by fading control.

it is to be noted that it is known per se to bring about automatic contrast variation as a function of the fading control voltage. It is. thus ensured that with weak input signals, which frequently contain strong interference, for example noise, the contrast is decreased and hence the interference is made active less strongly. With strong input signals, the contrast then increases to an extent such that the desired improved reproduction is obtained. fiince the sound intensity is maintained at least approximately constant by the fading control, variation in sound intensity substantially does not occur, however, and more particularly there is no contrast variation as a function of the sound intensity control.

According to the invention, the contrast variation is preferably provided continuously as a function of the sound intensity control. This may be achieved in a simple manner by coupling a control potentiometer of suitable characteristic, included in a known circuit for contrast variation, to the sound intensity potentiometer.

The extent of contrast decrease with low sound intensity of reproduction and the extent of contrast increase high sound intensity of reproduction may be adjustable. it is thus possible to match. the reproduction of Weak signals to the strength of the ambient noise and also to ary the contrast with high signal strength in accordance with personal taste.

For decreasing the sound intensity, for example with a telephone call or the like, it is known. to use a commutator, for example a key switch (piano key). In the device according to the invention, this device may also be advantageously used, a particularly simple device being obtained if said contrast variation is coupled to this switch. As a matter of fact, a device for continuous variation may be provided in addition.

For the contrast variation use is preferably made of a bridge. circuit known per. se, which includes at least a resistor, for example an incandescent lamp, the value contact arms of commutators 14 of which is dependent upon the load and more particularly upon the mean current. By means of continuous variation of its fixed resistors, it is possible to use the same bridge circuit for both contrast decrease and contrast increase. More particularly when using load-dependent resistors having values which increase with increasing load, the fixed resistors of the bridge for the contrast decrease must be larger than, and for the contrast increase smaller than the value of the associated load-dependent resistors with the highest or smallest load.

In order that the invention may be readily carried into efiect, it will now be described in detail, by way of example, with reference to the accompanying drawing.

In FIG. 1, the low-frequency oscillations obtained in areceiver or a pre-amplifier are supplied in a manner known per se to terminals 1 of a potentiometer comprising resistors 2 and 3. The extremities of resistor 2 are connected to the contacts of a commutator 4, the contact arm of which is connected to the input of an amplifier 5. The other input terminal of amplifier 5 is connected to the (lower) terminal 1 coupled to resistor 3.

The oscillations occurring in the output of amplifier 5 are supplied to the primary winding of a transformer 6, the secondary winding of which feeds a known bridge circuit for contrast variation, which, according to the invention, is specially proportioned and of the commutatable type.

The Y branches of this bridge circuit include incandescent lamps 8 and 9 and resistors 10 and 11, respectively. A loudspeaker 13 is connected between the connec tions of incandescent lamps 8 and 9 which are remote from the secondary winding of transformer 6. The arrangement according to the invention is so proportioned that the signals in the circuit shown are reproduced by loudspeaker 13 with decreased sound intensity, whilst also considerable decrease in contrast occurs.

Such was the case in an embodiment comprising a loudspeaker 13, which had an impedance of 800 ohms and was proportioned for a low-frequency power of from 3 to 6 watts. The secondary winding of transformer 6 was matched to an impedance of about 800 ohms. The fixed resistors 10 and 11 each had a value of about 1000 ohms. Incandescent lamps 8 and 9 for 60 volts, 50 111 amps. nominal load were used as resistors dependent upon the load and more particularly upon the mean current. Said incandescent lamps had a resistance of 200 ohms in the cold state (smallest signal strength) and a resistance of 1200 ohms in the hot state (highest load).

With this proportioning, the bridge if not loaded is out of balance due to the considerable difierence in partial' resistances and the low-frequency signal oscillations of small amplitudes can reach the loudspeaker substantially without attenuation. However, with high load (sound intensity peaks), the bridge approaches its condition of equilibrium, so that only a comparatively small portion of the power supplied can reach the loudspeaker 6. Obviously in this case the desired contrast decrease occurs. The mean sound intensity of reproduction (amplification) is then decreased, for example to one quarter, also by the action of potentiometer 2, 3.

In order to eliminate the contrast compression in the case of a' comparatively high volume of reproduction, and are connected to the junction of incandescent lamp 8 and loudspeaker 13 and to the junction of incandescent lamp 9 and loudspeaker 13, respectively.

When the switches 4, 14 and 15 are commutated by the common coupling shown in dotted line, the incandescent lamps 8 and 9 are bridged via lines 16 and 17 and the connection between loudspeaker 13 and the resistors 10 and 11 is interrupted, so that loudspeaker 13 is connected directly to transformer 6 and reproduces the full output.

The desired variation in mean sound intensity of reproduction, which is already obtained in itself by the 4. action of the commutators 14, 15 and the bridge circuit, may still be intensified due to the commutator 4, for comparatively low sound intensity of reproduction, making contact with the junction of resistors 2 and 3, as shown in FIG. 1, so that the input voltage of amplifier 5 is also decreased. For high sound intensity of reproduction, the input of amplifier S is connected directly to the terminals 1.

If in the bridge circuit above described, comprising the incandescent lamps 8 and 9 and the resistors 10 and ill, the value of the fixed resistors 10 and 11 is reduced below the resistance value of the incandescent lamps in the unloaded condition, for example to ohms, the bridge is approximately balanced with small load, sothat small signal amplitudes supplied are attenuated even more strongly. With high load, however, the oscillations reach loudspeaker 13 without any appreciable damping, a contrast increase thus occurring. By means of a correspondingdesign of the commutators 14, 1S and 4 with a plurality of switch positions, it is possible more particularly with the use of the same load-dependent elements 8 and 9 to ensure that contrast decrease occurs with low mean sound intensity of reproduction, that contrast variation does not occur with normal sound intensity, and that contrast increase occurs with a high mean sound intensity of reproduction.

The commutators 14, 15 and possibly 4 may be coupled to a piano key and operated if the sound intensity is to be considerably reduced, whereas the bridge is switched off with reproduction of high sound intensity.

The bridge may be proportioned, more particularly due to its resistors being switched into circuit in series or in parallel with the loudspeaker 13 included in the diagonal branch, so that upon switching in the contrast decrease, the decrease in sound strength of reproduction, which is also desired, takes place without the presence of a commutator 4. It is thus ensured that the bridge circuit, which usually has an optimum action only in a given range of amplitudes, is always controlled completely, the sound intensity of reproduction or the effective amplification being decreased only at its output.

The resistors of the bridge and more particularly also the resistors included in the loudspeaker branch may be completed in known manner by frequency-dependent impedances, preferably capacitors, so that the contrast be tween the high and more particularly the low notes (physiological volume characteristic) as desired with low sound intensity of reproduction is obtained at the same time.

The extent of contrast variation may also be controlled by means of an auxiliary resistor 8, preferably adjustable, which is connected in parallel with the load-dependent resistors 8 and 9, respectively. The resistance value of the load-dependent branch of the bridge is thus limited upwards, so that the condition of equilibrium is approached more slowly and the great amplitudes are influenced less strongly- The extent of contrast variation may alternatively be controlled by means of an auxiliary resistor 8", preferably adjustable, which is connected in series with the loaddependent resistor, an increased resistance of this branch then occurring when the load is smaller so that the bridge is then already closer to its condition of equilibrium. The resultant contrast decrease with great amplitudes is thus also reduced.

By means of the resistors 8' and 8", it is possible more particularly to influence also the characteristic of the contrast curve, that is to say, the relationship between the strength of the input signal and the sound intensity of reproduction.

By choosing the resistors 10 and 11 in the form of potentiometers and possibly switching into circuit adjustable Potentiometers, it is possible to obtain a continuous variation upon transition from contrast decrease to contrast anemia a) increase, or conversely. An example of such an embodimom is shown in FIG. 2.

In this embodiment, the input oscillations at the terininals 1 are supplied to a continuously variable potentiometer 3', the input of an amplifier 5 being included between its sliding contact and lower extremity. Connected to its output is a bridge circuit comprising the currentdependent resistors 8, 9 and a potentiometer device 21, 21', 22, 24, 24', 25, the loudspeaker 13 being included in the diagonal of this bridge.

The load-dependent resistors 8 and 9 each have a resistance of about 200 ohms in the absence of a signal and a resistance of about 1200 ohms in the presence of a strong signal.

The upp r port n of h Po en iomet rs 2 a 24 p to theirtappings have approximately a resistance value which is less than the smallest value of the load-dependent resistors 8 and 9, resp. in the absence of load and is, for example, 150 ohms.

If, in the position of the switches as shown, the fixed tappings are connected to the output terminals of amplifier 5 and the sliding contacts are connected to the upper extremities of the potentiometers 21 and 24, the bridge is approximately balanced with small amplitudes and unbalanced with great amplitudes, so that a contrast increase occurs. The contrast increase becomes less and less when the sliding contact is moved downwards and is nil when the fixed tapping is reached, a direct connection then existing between the loudspeaker 13 and the output of amplifier 5.

The commutators 22 and 25 are actuated by coupling devices 23 and 26, when the sliding contacts upon being moved further downwards pass beyond the fixed tappings and make contact with the potentiometer parts between the lower terminal connection and the fixed tapping. The commutator contact arms of the switches 22 and 25 are then connected to the lower terminal connections of the potentiometers 21 and 24, the lower parts of these potentiometers, the value of which may be, for example, from 2000 to 4000 ohms, being included in the bridge.

The bridge circuit is then greatly brought out of balance with small signals and more approaches its balance with increasing signal strength, resulting in a contrast decrease which is stronger as the tappings are moved further downwards. By means of fixed resistors 21 and 24 between the lower terminal connections of potentiometers 21, 24 and the associated switching contacts the remaining resistors included in these arms are limited to a value higher than the highest value of the load-dependent resistors 8 and 9 with high signal strength. With the abovementioned values of resistors 8 and y, the resistors 21 and 24 may have values of, for example, 1500 ohms. A contrast compression is then obtained in the other extreme positions of the slidable tappings.

The extent of contrast decrease is reduced when the sliding contacts are again moved upwards. When the fixed tapping is passed, the commutator 22 is switched back by the coupling devices 23, 26, so that the abovementioned condition is restored.

The described device thus permits of obtaining an adjustable contrast decrease or contrast increase in a comparatively simple manner.

As shown in dotted line in FIG. 2, a coupling exists between the contrast potentiometers 21, 24 and the sound intensity potentiometer 3, which coup-ling is such that a contrast decrease occurs upon adjustment to a lower sound intensity and a contrast increase occurs upon adjustment to a higher sound intensity, the transition taking place continuously. The coupling between the potentiometers may alternatively be adjustable, so that an independent after-control of the two devices is possible, in order to obtain better matching to the personal taste.

What is claimed is:

1. An amplifier for an audio frequency signal, comprising input means for said audio signal, output for audio utilization means, means arranged between said input and output means for controlling the volume level of said signal at said output means, an actuator for said control means, means responsive to said signal for compressing the volume of said signal at said output means, an actuator for said compressing means, said compressing means being adjustable to a first condition imparting a given compression characteristic to said signal at said output means and to a second condition imparting to said signal at said output means a less compressed characteristic relative to said first characteristic, said volume level control being adjustable to a first condition producing an audio signal of given means intensity at said output means and adjustable to a second condition pr0- ducing an audio signal of substantially greater mean intensity at said output means, means for actuating simultaneously said compressing means and said volume level control selectively to said first positions at Which an audio signal of said given mean intensity is produced at said output means and the said compressing means imparts said given compression characteristic to said signal and to said second positions at which an audio signal of substantially greater mean intensity is produced at said output means and the said compressing means imparts said relatively less compressed characteristic to said signal, said compressing means comprising a temperature responsive element and a plurality of resistors arranged in bridge formation, means applying said audio signal to said bridge whereby the temperature and resistance of said element is determined by the intensity of said audio signal, and means comprising said resistors for adjusting the extent of contrast variations produced by said compressing means.

2. An amplifier for an audio frequency signal, comprising input means for said audio signal, output means for audio utilization means, means arranged between said input and output means for controlling the volume level of said signal at said output means, an actuator for said control means, means responsive to said signal for compressing the volume of said signal at said output means, an actuator for said compressing means, said compressing means being adjustable to a first condition imparting a given compression characteristic to said signal at said output means and to a second condition imparting to said signal at said output means a less compressed characteristic relative to said first characteristic, said volume level control being adjustable to a frst condition producing an audio signal of given mean intensity at said output means and adjustable to a second condition producing an audio signal of substantially greater mean intensity at said output means, means for actuating simultaneously said compressing means and said volume level control selectively to said first positions at which an audio signal of said given mean intensity is produced at said output means and the said compressing means imparts said given compression characteristic to said signal and to said second positions at which an audio signal of substantially greater mean intensity is produced at said output means and the said compressing means imparts said relatively less compressed characteristic to said signal, said compressing means comprising a temperature responsive element and a plurality of resistors arranged in bridge formation, means applying said audio signal to input terminals of said bridge whereby the temperature and resistance of said element is determined by the intensity of said audio signal, the arm of said bridge including said element comprising a potentiometer having fixed and adjustable taps, means connecting a bridge input terminal to the contact arm of a switch the switching contacts of which are connected to said fixed tap and one end of said potentiometer, and means coupling said adjustable tap to said volume level control whereby further contrast decrease occurs upon adjustment to a lower '7 sound intensity and a further contrast increase occurs upon adjustment to a higher sound intensity.

3. An amplifier as claimed in claim 2, comprising a bridge circuit in which the value of the signal responsive resistor increases with increasing load, characterized in that, starting from the fixed tapping, that part of the potentiometer, the terminal connection of which is connected to the commutator, has a resistance value higher than the highest value of the signal responsive resistor, the other part of the potentiometer having a resistance value less than the smallest value of the signal responsive resistor. v

4. An amplifier as claimed in claim 3, characterized in that a fixedresistor is included between the terminal connection of the potentiometer and the switching contact, so that in the extreme position of the adjustable tapping, the resistance value of this arm is higher and preferably only little higher than the highest value of the load-dependent resistor.

5. An amplifier as claimed in claim 3, characterized in that the said switch is actuated when the adjustable tapping is moved beyond the fixed tapping, thus making electrical contact with the part of the potentiometer between the terminal connection and the fixed tapping, the switching contact arm being connected to the terminal of the potentiometer.

6. An amplifier as claimed in claim 2, characterized in that the adjustable tapping on the potentiometer is coupled to an adjusting member for the continuous volume control, the resistance value between the adjustable tapping and the terminal connection of the potentiometer being decreased upon adjustment to a lower sound in tensity.

7. An amplifier as claimed in claim 6, characterized in that-the relative position of the coupled controls is adjustable.

References Cited in the file of this patent UNITED STATES PATENTS H Rockwell Aug. 29, 1939 Canada Sept. 25, 1956 

